In the paper and plastic film industries, a dancer roll is widely used as a buffer between first and second sets of driving rolls, or first and second nips, which drive a continuous web. The dancer roll, which is positioned between the two sets of driving rolls, may also be used to detect the difference in speed between the first and second sets of driving rolls.
Typically, the basic purpose of a dancer roll is to maintain constant the tension on the continuous web which traverses the span between the first and second sets of driving rolls, including traversing the dancer roll.
As the web traverses the span, passing over the dancer roll, the dancer roll moves up and down in a track, serving two functions related to stabilizing the tension in the web. First, the dancer roll provides a damping effect on intermediate term disturbances in the tension in the web, e.g. disturbances that last more than 10 seconds. Second, the dancer roll temporarily absorbs the difference in drive speeds between the first and second sets of driving rolls, until such time as the drive speeds can be appropriately coordinated.
Typically, the dancer roll is suspended on a support system, wherein a generally static force supplied by the support system supports the dancer roll against an opposing force applied by the tension in the web and the weight of the dancer roll. So long as the tension in the web is constant, the dancer roll remains generally centered in its operating window on the track.
When the web encounters an intermediate or long term tension disturbance, temporarily increasing or decreasing the tension in the web, the imbalances of forces on the dancer roll cause translational movement in the dancer roll to temporarily restore the tension, and thereby the force balance. So when difference in the speeds of the first and second sets of drive rolls tend to accord a change in the web tension, the dancer roll temporarily maintains the tension.
Thus, the dancer roll generally stabilizes the tension in the web, by compensating for temporary changes in the operating tension. While the dancer roll, as conventionally used, provides valuable functions, it also has its limitations.
To the extent the tension disturbance is long term, such as a speed imbalance in the sets of driving rolls, the function of the dancer roll is only temporary, to accommodate the disturbance until resolution is addressed at the source by changing driving speeds. Such changing of the drive speeds is commonly known.
Conversely, to the extent the tension disturbance is short term, such as no more than 10 seconds, the mass and corresponding inertia of the dancer roll prevent the dancer from providing a meaningful response based on the static forces, during the period of existence of the disturbance. The response time based on the gravitational acceleration provided by the static forces is simply too slow to overcome the inertia of the dancer system in time to effectively counter such short-term tension disturbances.
It is known to provide an active drive to the dancer roll in order to reduce the response time in a static system, wherein the web is held under tension, but is not moving along the length of the web, whereby the dynamic disturbances, and the natural resonance frequencies of the dancer roll and the web are not accounted for, and whereby the resulting oscillations of the dancer roll can become unstable. Kuribayashi et al, "An Active Dancer Roller System for Tension Control of Wire and Sheet." University of Osaka Prefecture, Osaka, Japan, 1984.
However, it is not known to provide an active dancer roll in a dynamic system wherein dynamic variations in operating parameters are used to calculate variable active response force components for applying active and variable forces to the dancer roll, and wherein appropriate gain constants are used to compute effective response time without allowing the system to become unstable.
It is an object of the invention to provide methods and apparatus for controlling tension in a moving web, using a dancer roll, applying to the dancer roll a force having an active component, and adjusting the value of the active component at least one time per second.
It is another object to provide such methods and apparatus for controlling tension in a moving web, including sensing tension with sufficient frequency to identify a tension disturbance which exists for 10 seconds or less, and adjusting the active component of the force on the dancer roll at least five times during the existence of the tension disturbance.
Another object is to provide such methods and apparatus, including adjusting the value and direction of the active force component according to the equation EQU T*.sub.dancer =r[F.sub.d static +b.sub.a (V*.sub.p -V.sub.p)+k.sub.a (F*.sub.c -F.sub.c)].
A related object is to provide methods and apparatus for controlling the translational movement of the dancer roll such that the dancer roll, itself, creates controlled temporary and desirable tension disturbances.
Another objective is to provide methods and apparatus for providing such controlled tension disturbances repetitiously.